Cellulose esters having a degree of substitution less than 3 (i.e., less than fully substituted) find wide application in commerce. These materials serve many markets such as molding plastics, clear sheets, filter tow, and as coatings polymers, to name a few forms and applications. Methods for their economical and selective preparation are clearly desirable.
Polymers which have affinity for water are of great commercial interest. Water-absorbent polymers, such as poly(acrylates), are used commercially in diapers, absorbent pads, and similar high-volume consumer goods. Water-soluble polymers also find widespread use in the marketplace. They are used in foods, oilfield applications, cosmetics, and pharmaceuticals, to cite a few examples. It is clear, therefore, that new and superior processes for the manufacture of polymers with high water affinity may be of considerable benefit.
It is well known in the art that cellulose acetates with low degree of substitution (DS, i.e., the number of substituents per anhydroglucose ring) have high affinity for water. C. J. Malm (British Patent 356,012 (1929)) discloses the preparation of cellulose monoacetate (CMA) by the sulfuric acid-catalyzed hydrolysis of cellulose triacetate (CTA) in aqueous sulfuric acid. The product, having a DS of 0.6-0.8 acetyls, was soluble in water. This necessitated isolation by addition of a nonsolvent. Other drawbacks of the Malm procedure include the long reaction times and the necessity for continuous or sequential addition of water to maintain reaction rates, resulting in a dilute reaction mixture and difficulties in recovery of by-product acetic acid. Neutralization of the catalyst at the end of the reaction affords sulfate salts, which can be difficult to separate from the polymer. Similar work by C. L. Crane (U.S. Pat. No. 2,327,770 (1943)) disclosed that cellulose diacetate could be hydrolyzed in aqueous acetone or aqueous alcohol using sulfuric acid catalyst to afford a water-soluble CMA. This process suffers shortcomings which are similar to those of the Malm process cited above.
In U.S. Pat. No. 2,005,383, T. F. Murray and C. J. Staud disclose the use of zinc iodide in ethanol to solvolyze CTA. This process afforded a product with DS about 1.75, required long reaction times, and consumed large amounts of zinc iodide (10 parts ZnI per part CTA). Even with this amount of zinc iodide, 40 hours reaction time was required to produce the product of DS 1.75.
In U.S. Pat, 2,836,590 (1958) H. W. Turner discloses high temperature (&gt;180.degree.C.) alcoholysis of cellulose acetate without the use of catalysts. At the temperatures disclosed by Turner, cleavage of the 1,4-glycosidic linkages of the cellulose ester backbone competes with the desired deacylation.
A different approach to CMA was taught by M. Diamantoglou, A. Brandner, and G. Meyer in U.S. Pat. No. 4,543,409 (1985). They acetylated cellulose in homogeneous solution (in N,N-dimethylacetamide (DMAC) containing lithium chloride). The product was a cellulose monoacetate by its DS, but was not soluble in water. There are serious concerns associated with the use of the toxic and expensive DMAC as a commercial reaction solvent.
There is, therefore, a need for a process to prepare cellulose acetates with low degree of substitution, and possessing high affinity for water. The process must use solvents which are inexpensive and easily recycled. It must employ catalysts which are either powerful enough to be used in small amounts or inexpensive enough to be used in large amounts when necessary. The process must allow for easy and economical product isolation and simple and economical recycle of solvents. It must require economically short reaction times, be reliable and repeatable, and use practical reaction temperatures.